Rationale: books yaml files are not really configuration for the app,
they're really the source of data for the app, kind of like a database
would be. I think moving them up to the top level makes for a more
intuitive place to find these files. Plus, if we want extensions
maintainers to be adding their own books to the system, this new
organization scheme makes the process of adding a book yaml file just a
tad easier for people who are unfamiliar with the inner workings of
this app.